In many data processing systems more than one clock source may be utilized for providing clock pulse signals at different frequencies, i.e., at different pulse repetition rates. During operation of such systems the system clock signal must sometimes be switched from one clock source to another.
In many such systems the switching process introduces spurious signals at the clock output line, or lines, which supply the clock pulses to various elements of the data processing system, the presence of such spurious signals (often called "glitches") providing an incorrect operation of the system. In order to avoid such a problem, switching techniques in the past have often required relatively sophisticated and complex circuitry for eliminating the spurious signals, the use of such circuitry increasing the overall costs of the clock selection circuitry.
It is desirable, therefore, to achieve such switching in an effective manner so as to avoid such spurious signals and to do so utilizing much simpler and less costly logic circuitry than has been used in the past.